title: P680 (P(D1)P(D2)) and Chl(D1) as alternative electron donors in photosystem II core complexes and isolated reaction centers.
abstract: Low temperature (77-90 K) measurements of absorption spectral changes induced by red light illumination in isolated photosystem II (PSII) reaction centers (RCs, D1/D2/Cyt b559 complex) with different external acceptors and in PSII core complexes have shown that two different electron donors can alternatively function in PSII: chlorophyll (Chl) dimer P(680) absorbing at 684 nm and Chl monomer Chl(D1) absorbing at 674 nm. Under physiological conditions (278 K) transient absorption difference spectroscopy with 20-fs resolution was applied to study primary charge separation in spinach PSII core complexes excited at 710 nm. It was shown that the initial electron transfer reaction takes place with a time constant of ~0.9 ps. This kinetics was ascribed to charge separation between P(680)* and Chl(D1) absorbing at 670 nm accompanied by the formation of the primary charge-separated state P(680)(+)Chl(DI)(-), as indicated by 0.9-ps transient bleaching at 670 nm. The subsequent electron transfer from Chl(D1)(-) occurred within 13-14 ps and was accompanied by relaxation of the 670-nm band, bleaching of the Pheo(D1) Q(x) absorption band at 545 nm, and development of the anion-radical band of Pheo(D1)(-) at 450-460 nm, the latter two attributable to formation of the secondary radical pair P(680)(+)Pheo(D1)(-). The 14-ps relaxation of the 670-nm band was previously assigned to the Chl(D1) absorption in isolated PSII RCs [Shelaev, Gostev, Nadtochenko, Shkuropatov, Zabelin, Mamedov, Semenov, Sarkisov and Shuvalov, Photosynth. Res. 98 (2008) 95-103]. We suggest that the longer wavelength position of P(680) (near 680 nm) as a primary electron donor and the shorter wavelength position of Chl(D1) (near 670 nm) as a primary acceptor within the Q(y) transitions in RC allow an effective competition with an energy transfer and stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as the primary electron donor and Pheo(D1) as the primary acceptor cannot be ruled out, the 20-fs excitation at the far-red tail of the PSII core complex absorption spectrum at 710 nm appears to induce a transition to a low-energy state P(680)* with charge-transfer character (probably P(D1)(δ+)P(D2)(δ-)) which results in an effective electron transfer from P(680)* (the primary electron donor) to Chl(D1) as the intermediary acceptor.
authors.affiliation: N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia.
authors.forename: I V
authors.initials: IV
authors.lastname: Shelaev
authors.forename: F E
authors.initials: FE
authors.lastname: Gostev
authors.forename: M I
authors.initials: MI
authors.lastname: Vishnev
authors.forename: A Ya
authors.initials: AY
authors.lastname: Shkuropatov
authors.forename: V V
authors.initials: VV
authors.lastname: Ptushenko
authors.forename: M D
authors.initials: MD
authors.lastname: Mamedov
authors.forename: O M
authors.initials: OM
authors.lastname: Sarkisov
authors.forename: V A
authors.initials: VA
authors.lastname: Nadtochenko
authors.forename: A Yu
authors.initials: AY
authors.lastname: Semenov
authors.forename: V A
authors.initials: VA
authors.lastname: Shuvalov
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D004579:Electron Transport; D007700:Kinetics; D010782:Photolysis; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature; D013816:Thermodynamics
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll